German Pat. No. 606,027 discloses a process for producing indolines and/or indoles by catalytic gas-phase reaction of 2-(o-aminoaryl)ethanols. A list of the catalysts usable in this process includes Cu, Co, Ni, Ag, Fe, Pd, Pt, Mo, W, Pb, Mn, Al, Ti, Zr, Cr and Th, and CuCO.sub.3 is mentioned as a specific example capable of producing indole as the main product. However, none of these catalysts are completely satisfactory for insutrial purposes primarily because their activity is reduced during run.
Methods for producing indoles from indolines by dehydrogenation reaction are also known and they include the use of a Raney nickel catalyst [Acta Chim. Acad. Sci. Hung., 54, 167 (1967)], the use of a chromium or copper-chromium catalyst [Roczniki, Chem., 38, 507 (1964)] and the use of a manganese dioxide catalyst [J. Org. Chem., 29, 1540 (1964)]. However, these catalysts have the disadvantages of low indole yields and selectivities.
Unexamined Published Japanese Patent Application No. 142063/77 discloses a method of producing indoles by heating indolines or 2-(o-aminoaryl)ethanols together with at least one acidic compound selected from the group consisting of nitric acid, nitrous acid, an acidic sulfate salt and an acidic sulfite salt. This method is not industrially feasible, either, since it consumes a large amount of catalyst.
Japanese Patent Publication No. 7619/85 proposes a one-step production process of indole by heating o-(2-nitrophenyl)ethanol in the liquid state in the presence of hydrogen and a reducing catalyst, in order to improve the industrial disadvantage of reduced catalytic activity inevitable in a gas-phase process for producing indole from o-(2-nitrophenyl)ethanol in a single step (Japanese Patent Publication No. 20778/74). According to the Publication, the catalyst used in this process experiences a relatively small activity loss even if a large amount of the starting material is processed. However, a large quantity of catalyst is necessary for the purpose of cyclic use and the yield of indole produced per unit amount of catalyst is low. The present inventors repeated this prior art method and found that it is not industrially feasible because of the very low yield of indole obtained (see Referential Examples 1 to 3 given later in this specification).